1. Field of the Invention
The present invention relates to the treatment of substrates with a plasma jet. More particularly, the present invention relates to an apparatus for treating substrates with a plasma jet.
2. Description of the Prior Art
The application of plasma technology for treating substrates, such as wafers, is viewed as a replacement for liquid chemical treatment of substrates. U.S. Pat. No. 5,474,642 describes an apparatus for the plasma treatment of substrates. The apparatus described therein includes a means to generate a plasma jet at atmospheric pressure, and means to control both the cross-sectional size of the jet and the relative speed of movement. The apparatus consists of a closed process chamber, one or more substrate holders, a carousel, a plasma jet generator, and a device for moving the plasma jet generator all of which are located inside the process chamber. The process chamber is provided with system for providing a reactive gas and for exhausting gas. The carousel consists of angle drive and arms firmly connected with angle drive shaft. The substrate holders are attached to the ends of the carousel arms. Every substrate holder is made as a flat platform with vacuum chucks to hold a substrate. The plasma jet generator is directed to the flat platform of substrate holder. The device for moving plasma jet generator causes the plasma jet generator to move along the radius of the carousel parallel to carousel plane with a range of movement which is greater than the size of the substrate. The disadvantages of this apparatus is low throughput for the treatment of large size substrates and quality of substrate treatment.
In this apparatus, the plasma jet footprint is less than the substrate size. The substrate is held on the substrate holder flat platform by vacuum chucks. The carousel is rotated and the plasma jet periodically crosses the substrates on the substrate holder. After every crossing of the plasma jet, only part of the substrate is treated by the plasma jet. The treated part of the substrate looks like an arc with the width equal to the width of the plasma jet footprint having a radius equal to distance between the plasma generator and the carousel rotation axis. After every carousel revolution, the plasma generator is shifted along the carousel radius a distance which is less than the plasma jet footprint size to provide treatment of another portion of the substrate. With this shift, the plasma jet overlaps over the previously treated portion. The above described procedure is repeated until substrate surface has been treated completely.
When the substrate is passed across the plasma jet, the treated part of the substrate is heated and so after every passing of the plasma jet over the substrate, the substrate has to be cooled down to near its pre-treatment temperature to avoid heat accumulation in the substrate which can cause thermal damage of the substrate material. The duration .tau..sub.cool of cooling (10-30 seconds) is much longer than duration heating .tau..sub.pass of every substrate pass (0.05-0.15 seconds). So the duration T.sub.treat of whole substrate treatment and correspondingly the throughput G of the apparatus may be described as: ##EQU1## where M is the quantity of substrate passes to achieve required result of the treatment at any point of substrate surface, e.g. thick layer etch removal by consequent removal of M thin layers;
K is the quantity of necessary substrate passes to treat whole surface of the substrate with diameter D by plasma jet with cross-section P and with pass overlapping coefficient k.congruent.0.7-0.3.
N is the quantity of simultaneously treated substrates on the carousel.
As it is seen from equations (1) and (2), as substrate size D becomes larger, a greater quantity of passes K are necessary to treat whole substrate surface and as a result the throughput G is reduced dramatically.
As only part of the substrate becomes heated after every crossing of the plasma jet over the substrate, high temperature gradients in the substrate result and in turn induce high thermo-elastic stress in the substrate which may cause undesirable deformation and damage, especially in the case where fragile mono-crystal semiconductor substrates are being treated. Moreover, for etching processes, the etch products removed from the treated part of the substrate may interact with cold surface of adjacent non-treated parts of the substrate and cause re-deposition and contaminations of the substrate. For plasma enhanced chemical vapor deposition, the non-uniform temperature distribution of the substrate surface may cause non-uniform material properties of deposited films and therefore unacceptable quality.